Method for making a surface-mount technology plastic-package ball-grid array integrated circuit

ABSTRACT

A method of making a surface-mount technology plastic-package ball-grid array integrated circuit assembly. According to the method, a surface-mount ball-grid array package is provided for an integrated circuit assembly. The array package has a circuit chip recess through which vias extend, opening on a bottom surface of the array package. A peripheral portion of the array package is defined around the vias formed in the recess. The array package is held on a vacuum chuck by applying vacuum to the peripheral portion of package. An adhesive material is then placed in the recess to extend partially through the vias. A circuit chip is finally disposed in the recess on the adhesive material to complete the fabrication process. Substantially ambient pressure is communicated to the vias on the bottom surface of the array package to prevent the adhesive from being pulled through the vias by the applied vacuum.

FIELD OF THE INVENTION

The present invention is in the field of integrated circuits. Moreparticularly, the present invention is in the field of integratedcircuits having a plastic package using surface mount technology of theball grid array type. Still more particularly, the present inventionrelates to a surface-mount plastic package ball-grid array integratedcircuit which is manufactured using an improved method and apparatus toprovide both an improved yield of integrated circuits from themanufacturing process as well as an improved integrated circuit ofbetter reliability being less prone to damage and circuit failure duringa surface-mount solder reflow installation step of manufacture.

RELATED TECHNOLOGY

Integrated circuits having plastic packages of the surface-mountball-grid array type are known conventionally. That is, integratedcircuits are known which are available in plastic packages. The plasticpackages are configured to use surface-mount electrical contactsconnected to external circuitry, as opposed to depending on electricalcontact legs which are received into a socket or soldered intoconductive vias of a printed circuit board, for example. Still further,the conventional surface-mount technology of such packages is known touse the solder ball reflow technique of surface mounting the integratedcircuit package. In this mounting method an array of fine-dimensionsolder balls is positioned in a grid matching both the locations ofconductive vias (electrical contacts) of the integrated circuit package,and also matching the positions of corresponding electrical contacts ofa circuit board to which the integrated circuit is to be mounted. Thejuxtaposed circuit board and integrated circuit package with interposedsolder balls is then heated just sufficiently to reflow the solderballs, effecting the mounting and electrical connection of theintegrated circuit package to the circuit board. An adhesive or pottingcompound may additionally be used to secure the integrated circuit tothe circuit board.

However, with the conventional technology, when the integrated circuititself is placed into its package during the manufacturing process, athermally conductive, electrically non-conductive adhesive is normallyused to secure the integrated circuit die or chip into a recess of abase portion of the package. During this automated manufacturingprocess, the integrated circuit package is held in place on a vacuumchuck by applied vacuum and atmospheric pressure. The thermallyconductive adhesive extends partially through vias of the package towardthe back surface thereof to provide a heat transfer path from theintegrated circuit toward the circuit board or heat sink upon which theintegrated circuit is disposed as installed. As will be understood, thisheat transfer path for cooling the integrated circuit chip is animportant feature because the plastic material of the package itself innot a particularly good heat conductor, and the circuit chip producesheat during operation which must be carried to ambient temperature.While some heat transfer occurs without the presence of the adhesive,this adhesive has been found to substantially facilitate heat transfer.In order to prevent at this stage of manufacture the thermallyconductive adhesive from being pulled through the vias by the vacuum ofthe vacuum chuck, these vias are closed on the back side of the packageby a non-conductive film or solder resist, for example. These closedvias must be opened later, however, before the finished package isexposed to the heating necessary for solder reflow during thesurface-mount attachment step of the packaged integrated circuit to thecircuit board.

Such step of opening the closed vias is necessary because the vias havesome void volume therein which may contain volatile materials. Forexample, a small amount of water moisture is sufficient if trapped inthe closed vias under the integrated circuit chip to expand from theelevated temperature of the solder reflow mounting process, and blisteror delaminate the plastic package of the integrated circuit. Theintegrity of the electrical connections within the package and of theintegrated circuit itself are normally too much in doubt after adelamination of the package to risk use of such a damaged integratedcircuit package. Of course, unpackaging, retesting, and repackaging theintegrated circuit chip are not economically feasible. Delamination ofthe plastic package also renders the integrated circuit unusable becausethe package is typically too distorted to provide reliable electricalconnections with the circuit board during the ball grid array attachmentprocess. Consequently, a package defect, such as a delaminationdescribed above, results in the entire package and integrated circuitchip (essentially a finished product with all expenses already sunk intoit) being scrapped.

In order to avoid the problem described above, conventionalmanufacturing methods include a step in which the non-conductive film orcoating of solder resist is punctured at the back side of the package.The puncturing of the film and opening of the vias to ambient provides avent path for any volatile materials which are vaporized during thesolder reflow step. Accordingly, blistering of the integrated circuitpackages by pressure caused from volatile materials trapped in the viasis avoided. However, this manufacturing step of puncturing the film orcoating of solder resist at the package back side adds another step andadditional cost to the manufacturing operation.

As a result of the problem described above, some consideration has beengiven and evaluation attempts have been made to manufacture theintegrated circuit package with the vias open both in the chip-receivingrecess and on the bottom surface of the package. However, this method ofmanufacturing the packaged integrated circuits caused the vacuum fromthe vacuum chuck to draw the adhesive from the chip-receiving recessonto the back surface of the package. This misplaced adhesive is acontaminant on the backside of the package and presents the risk ofinterference with the electrical connections which must be made to theelectrical contacts of the package using the solder ball grid array.Also, the adhesive may migrate into the manufacturing machinery andcontaminate successive packages moving through the manufacturingmachines. Finally, such misplaced adhesive may be sufficientlyconductive or may retain conductive dust or other contaminants inlocations between the electrical connections of the package so thatelectrical shorting results between these contacts.

SUMMARY OF THE INVENTION

In view of the deficiencies of the conventional technology outlinedabove, an object for this invention is to provide a method of making asurface-mount technology plastic-package ball-grid array integratedcircuit having vias in the chip-attach cavity or recess which are openat each end without a necessity for a separate manufacturing step toopen these vias after mounting of the circuit chip in the cavity so asnot to trap volatile materials therein which subsequently could causeblistering of the package during solder reflow for surface mounting ofthe packaged integrated circuit.

Further to the above, an object for this invention is to provide amanufacturing method and apparatus for making such a surface-mounttechnology plastic-package ball-grid array integrated circuit.

Accordingly, the present invention provides according to one aspectthereof, a surface-mount technology plastic-package ball-grid arrayintegrated circuit having vias in the chip-attach cavity or recess whichare open at each end without a necessity for a separate manufacturingstep to open these vias after mounting of the circuit chip in thecavity.

According to another aspect of the present invention, a vacuum chuck foruse in manufacturing the inventive surface-mount technologyplastic-package ball-grid array integrated circuit and which includes anisland of ambient pressure congruent with the open vias of the chippackage during the adhesive mounting of the integrated circuit chip inthis cavity is provided.

An advantage of the present invention resides in the simplified and lessexpensive inventive manufacturing process and apparatus which may beused to manufacture surface-mount technology plastic-package ball-gridarray integrated circuits, and in the improved and more reliable productwhich results from this manufacturing process.

These and additional objects and advantages will be apparent from areading of the following detailed description of a single exemplarypreferred embodiment of the invention taken in conjunction with thefollowing drawing Figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a plastic package surfacemount integrated circuit assembly with an integrated circuit chip andelectrical interconnection device;

FIG. 2 is a view similar to FIG. 1, but showing the bottom surface ofthe integrated circuit package;

FIG. 3 provides and enlarged fragmentary partially cross sectional viewof the integrated circuit package and circuit chip as they would appearat a particular step of manufacturing, and as disposed on an inventivemanufacturing apparatus according to the present invention; and

FIG. 4 provides a perspective view of the manufacturing apparatus seenalso in FIG. 3, with a portion of the apparatus broken away to betterillustrate the structure.

DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENT OF THE INVENTION

Viewing FIGS. 1 and 2 in conjunction, a surface-mount technologyplastic-package ball-grid array integrated circuit assembly 10 isdepicted in exploded view. The integrated circuit assembly 10 includes atwo-part package, generally indicated with the numeral 12. This package12 includes a base member part 14, which is a generally planar body ofplastic material which is square in plan-form, and which defines arecess 16 into which an integrated circuit chip 18 is receivable. Thebase 14 defines a plurality of vias or passages 20 opening on the floor22 of the recess 16, and also opening on the bottom surface 24 of thebase member 14, as is visible in FIG. 2.

Surrounding the recess 16, the base member 14 carries a plurality ofelectrical contacts 26. Typically the contacts 26 are disposed in astepped recess as shown in FIG. 1. Of course, the region of the basemember 14 carrying the contacts 26 could also be level with the floor 22of the recess 16. These contacts 26 each individually connectelectrically to corresponding ones of plural electrical contacts 28disposed on the bottom surface 24 of the base member 14, also as is seenin FIG. 2. The electrical contacts 28 may be essentially flush with thebottom surface 24 of the package 12, or may protrude or be recessedslightly. However, carried on the bottom surface 24 of the base member14 is a film or coating of solder resist material 30. This solder resistmaterial defines plural openings 32 corresponding to the plurality ofcontacts 28. In other words, each of the plural contacts 28 is disposedin a corresponding one of the plural openings 32, and is surrounded bythe coating or film 30 of solder resist material to be separated fromadjacent contacts by an interposed portion of the solder resist film orcoating. The solder resist film or coating 30 also defines a pluralityof openings 34 corresponding to the vias 20 so that these vias open onthe lower surface of the package 12 despite this solder resist film orcoating.

As will be seen, the package 12 receives the chip 18 into the recess 16,there to be secured by an adhesive material (not seen in FIGS. 1 or 2).Viewing FIG. 1, it is seen that the chip 18 includes pluralperipherally-disposed fine-dimension electrical contacts, generallyreferenced with the numeral 36 corresponding to the individual contactsof the plurality of contacts 26 carried on the base member 14. Typicallythe individual contacts 36 are electrically connected to theircorresponding individual contacts 26 by wire bonding 38 (see FIG. 3).The use of wire bonding connection technology is widely known in thesemiconductor integrated circuit packaging art.

Still viewing FIG. 1, it is seen that the package 12 includes a coverportion 52 which sealingly cooperates with the base portion 14 toprovide a sealed and protected environment for the integrated circuitchip 18. During manufacturing of the integrated circuit assembly 10, thepackage cover portion 52 is sealingly attached to the base portion 14,for example, by use of a potting compound or epoxy. As a result, theintegrated circuit chip 18 is environmentally protected once themanufacturing operations are completed to install the chip in thepackage 12 and to close this package. Alternatively, the cover portion52 may be replaced with a simple layer of epoxy or other pottingcompound to provide environmental closure for the package 12, and forthe integrated circuit chip 18 therein.

FIG. 2 shows by use of the dashed line 54 that outwardly of the recess16, an annular or peripheral area 56 is defined on the bottom surface 24of the package 12 within which the contacts 28 are disposed, but whichis closed with respect to fluid communication between the upper surface58 of the base member 14 and the lower surface 24 of this base member.Within the dashed line 54 (that is, within the surrounding peripheralarea 56), the vias 20 provide fluid communication between the recess 16and the bottom surface 24 of the package base member 14.

FIGS. 3 illustrates the integrated circuit assembly 10 at anintermediate stage of manufacture and disposed on a vacuum chuck 60,which is also illustrated in FIG. 4 without the package 12. In order tobetter illustrate salient features of the invention, FIG. 3 shows therecess 16 and chip 18 greatly enlarged in comparison with the size ofbase position 14. This size distortion is carried over into FIG. 4 forconsistency of illustration. At the stage of manufacturing depicted inFIG. 3, the base member 14 is disposed on the vacuum chuck 60 and isthere retained by an applied vacuum in conjunction with ambientpressure, as will be further explained. The base member 14 firstreceives into the recess 16 a quantity of an adhesive material 62initially in a semi-liquid state, and the integrated circuit chip 18 isthen disposed on this adhesive material 62 to be permanently retained inthe recess 16 when the adhesive 62 cures to a solid state. At this stageof manufacture, the wire bond 38 is not yet applied to electricallyconnect the circuit chip 18 to the base member 14.

Viewing FIG. 4, it is seen that the vacuum chuck 60 includes a basemember 64 having a floor portion 66 and an upwardly extending peripheralwall portion 68. The wall portion 68 terminates in a generallyhorizontal circumferentially continuous edge surface 70, which matchesthe size and shape of the outer extent of the peripheral area 56 of thepackage base member portion 14, recalling FIG. 2. This edge surface 70carries a peripheral outer sealing member 72, which is sealinglyengageable with the lower surface 24 of the package base 14 generally ata sealing line, the outer extent of which is indicated at 72', seen inFIG. 2. Downwardly through the center of the floor portion 66 opens avacuum conduit 74, which communicates to a controllable source ofvacuum, as is indicated by the arrow 76.

Within the wall 68 and upon floor 66 sets an island member, generallyreferenced with the numeral 78. This island member 78 similarly includesa floor portion 80 which is spaced above the floor 66 by an upwardlyextending wall portion 82. The wall portion terminates at an upper edgesurface 84, which matches the size and shape of the inner extent of theperipheral area 56 of the package base member 14, again recalling FIG.2. This edge surface 84 carries a circumferentially extending innerperipheral sealing member 86. The edge surfaces 70 and 84 are generallyat the same elevation or generally in a common plane so that the sealingmembers 72 and 86 are generally coplanar. As a result, peripheralsealing member 86 sealingly engages the lower surface 24 of the packagebase member 14 generally at a sealing line, the inner extent of which isindicated at 86', which is seen in FIG. 2. The sealing lines 72' and 86'are each disposed within the area 56 of the package base member 14.

Centrally through the floor portion 80 of the island member 78, anambient air conduit 88 opens downwardly and extends a distance withinthe vacuum conduit 74 to communicate through the wall of this latterconduit at 90 in order to outwardly communicate with an air filtermember 92. The conduit 88 thus communicates the internal portion ofisland member 78 with ambient air, as will be further explained. Becausethe floor 80 is spaced above the floor 66, the base member 64 and islandmember 78 cooperate to define a chamber 94 communicating with the vacuumsource 76. This chamber 94 communicates with a peripheral moat-like orgroove-like vacuum recess 96 defined between the two walls 68 and 82 viaa plurality of passages 98 formed in the latter wall portion. Withinthis peripheral vacuum recess 96, the island member 78 defines anambient air island or basin 100, recalling the ambient air conduit 88described above.

Viewing FIG. 3, it is seen that when the package base member 14 isdisposed on the vacuum chuck 60 in sealing cooperation with the sealingmembers 72 and 86, the peripheral vacuum recess 96 and ambient air basin100 are transformed into respective chambers, the former communicatingwith the vacuum source 76, and the latter communicating with the ambientair via the conduit 88 and filter 92. As FIG. 3 more particularly shows,with the package base member portion 14 disposed on the vacuum chuck 60and there retained by applied vacuum effective within the peripheralarea 56 between the sealing lines 72' and 86', recalling FIG. 2, novacuum is applied to the vias 20. Initially, the chip recess 16communicates with ambient air pressure by way of the vias 20. However,when the adhesive material 62 is placed in the recess 16, and the chip18 is placed on this adhesive, the vias 20 are closed. Under theseconditions, the communication with ambient pressure provided by theconduit 88 and filter 92 insures that ambient air pressure is maintainedin the chamber 100 (viewing FIG. 3) even if should there be only animperfect seal between the base member 14 and the sealing member 86.That is, a vacuum leak at the seal 86 will not pull down the pressure inchamber 100 to a vacuum.

Further to the above, the adhesive material 62 may extend downwardly inthe vias 20 under its own weight and surface tension to advantageouslyprovide a conductive heat transfer path from the chip 18 toward acircuit board or heat sink which will be adjacent to the surface 24 asthe package 10 is installed in its use environment. Understandably, theextent of downward flow of the adhesive 62 along the vias 20 can becontrolled quite precisely by controlling the rheology, viscosity,surface tension and amount of the adhesive which is placed into therecess 16 prior to placement of the chip 18. However, the vias 20 arenot exposed to vacuum during the manufacturing step illustrated in FIG.3, and the adhesive 62 is not drawn through the vias 20 and into thevacuum chuck 60 or onto the lower surface 24 of the integrated circuitassembly 10.

Still further, when manufacturing of the integrated circuit packageassembly 10 is complete, this package assembly is surface mounted to acircuit board using an array of solder balls at the contacts 28. Thesesolder balls are then heat reflowed to effect the necessary electricalconnections between the integrated circuit package 12 and the circuitboard on which it is to be mounted in its use environment. The open vias20 of the package assembly 10 cannot trap volatile materials which mightbe vaporized during this solder ball heat reflowing process to blisterthe package 12. Consequently, the problem of blistering and destructionof the completed conventional integrated circuit package assembliesduring this surface-mounting step described above with respect toconventional packages is avoided by the present invention. Thisadvantage is achieved by the present invention also without thenecessity of a separate manufacturing step to open previously closedvias after the chip 18 has been adhesively secured into the recess 16.

While the present invention has been depicted, described, and is definedby reference to a particularly preferred embodiment of the invention,such reference does not imply a limitation on the invention, and no suchlimitation is to be inferred. The invention is capable of considerablemodification, alteration, and equivalents in form and function, as willoccur to those ordinarily skilled in the pertinent arts. The depictedand described preferred embodiment of the invention is exemplary only,and is not exhaustive of the scope of the invention. Consequently, theinvention is intended to be limited only by the spirit and scope of theappended claims, giving full cognizance to equivalents in all respects.

I claim:
 1. A method of making a surface-mount technologyplastic-package ball-grid array integrated circuit assembly, said methodcomprising the steps of:providing for said integrated circuit assembly asurface-mount ball-grid array package which has a circuit chip recessfrom which through vias extend to open on a bottom surface of saidpackage; holding said package on a vacuum chuck by applying vacuum to aperipheral portion of said package around said vias; communicatingsubstantially ambient pressure to the openings of said vias on saidbottom surface of said package; placing an adhesive material in saidrecess to extend partially through said open vias; and disposing acircuit chip in said recess on said adhesive material.
 2. The method ofclaim 1 further including the steps of isolating said through vias fromsaid applied vacuum.
 3. The method of claim 2 further including thesteps of:configuring said vacuum chuck to include an outer wall portionupon which said integrated circuit assembly package is sealinglyreceivable; providing within said outer wall portion an island membercommunicating with ambient air; utilizing said island member tocommunicate ambient air pressure to the openings of said vias on thebottom surface of said package.
 4. The method of claim 3 additionallyincluding the steps of:configuring said island member to include a wallportion spaced within said out wall of said vacuum chuck; and sealinglycooperating said island member wall with said package at a sealing linecircumscribing said through vias to isolate said vias from said appliedvacuum.